1. Field of the Invention
The invention relates to an apparatus and a method for squelch gating a receiving signal.
2. Related Technology
In digital receivers the possibility to mute an audio output signal in case of disappearance of the carrier of the received audio signal or in case of a drop of the signal-noise-ratio in the received audio signal below a threshold is known from U.S. Pat. No. 6,047,170 A.
The technical realization for determining the measure of noise in a received signal is called squelch and is based on the continuous measuring of phase noise in the audio signal and the gating of the demodulated audio signal according the measured level of phase noise in comparison with specified thresholds.
FIG. 1 shows an apparatus for gating a received audio signal on the basis of a squelch signal characterizing the measure of phase noise in the audio signal.
The received sampled RF audio signal xn with its sampled cartesian components—sampled real component Re{xn} and sampled imaginary component Im{xn}—is converted in a Cordic-unit 1 to its polar components, sampled amplitude mn and sampled phase φn. In a differentiating unit 3 of a squelch detector 3 a sampled phase difference Δφn between a sampled phase φn and its succeeding sampled phase φn-1—determined in a delay unit 4 of the differentiating unit 3 on the basis of the sampled phase φn-1—is determined in subtracting unit 5 of the differentiating unit 3.
The sampled phase difference Δφn contains signal portions of low frequency, caused by frequency offset or frequency drift of the carrier in the received RF audio signal, and phase noise as signal portions of high frequency. To eliminate the signal portions of low frequency in the sampled phase difference Δφn a subsequent highpass filter 6 of the squelch detector 2—typically a 4th order IIR highpass filter—produces sampled phase differences Δφn comprising only phase noise relevant signal portions.
The signed sampled phase differences Δφn corresponding to phase noise are rectified in a subsequent squaring unit 7. The squaring unit 7 can alternatively be replaced by a device performing the absolute value of its input signal. For averaging the sampled unsigned RF phase differences pn over time a digital filter 8 having a proportional-integral characteristic concerning equation (1) represents the last signal processing unit of the squelch detector 2 delivering the sampled squelch signal sn at its output. In equation (1) fs represents the sampling rate and τ represents the time constant of the integral characteristic of the digital filter 8.
                              s          n                =                              s                          n              -              1                                +                                                    f                s                            τ                        ·                          (                                                p                  n                                -                                  s                                      n                    -                    1                                                              )                                                          (        1        )            
The sampled amplitudes mn of the RF receiving audio signal xn are demodulated in a demodulator 9. The demodulated audio signal yn is gated in a subsequent gating unit 10 on the basis of the determined sampled squelch signal sn and a specified threshold delivering a muted audio signal an to the outputting unit 11.
In an off→on→off-cycle of an audio transmission FIG. 2 displays the DC component of the carrier DC(mn) (curve 40), the rectified RF phase noise pn (curve 20) and the squelch signal sn (curve 30) in the apparatus for gating a received audio signal according to FIG. 1, whereby the time constant τ of the integrating digital filter 8 ms is 5 ms.
The time constant of the digital filter is typically between 5 ms and 10 ms. Thus random peaks in the RF phase noise pn being shorter than 5 ms to 10 ms are not averaged by means of the digital filter 8 with proportional-digital characteristic leading to a squelch signal sn with random peaks. Such peaks in the squelch signal sn drops below respectively rises above the specified threshold(s) and disadvantageously mutes the received audio signal for a short period. Especially in case of a squelch signal sn rising above respectively dropping below a specified threshold leading to switch off respectively switch on of the received audio signal random peaks in the squelch signal sn occurring shortly after the switch reverses the switch of the received audio signal.
Prolongation of the time constant τ of the integrating digital filter 8 for avoiding such unwanted reversals of switches in the audio signal in case of peaks in the squelch signal would deteriorate the time behavior of the integrating digital filter and thus the time behavior of the squelch detector. The worse reaction of the integrating digital filter would undesirably delay the squelch signal sn in comparison with an on→off-step or an off→on-step of the carrier in the audio transmission (see in FIG. 2 the delay of the squelch signal sn (curve 20) in comparison with an off→on-step of the DC component of the carrier DC(mn) (curve 40)).